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# YUI Library Examples: YAHOO Global Object: Creating a Composition-Based Class Structure Using `YAHOO.lang.augmentProto`

# YAHOO Global Object: Creating a Composition-Based Class Structure Using `YAHOO.lang.augmentProto`

You may be familiar with this method as `YAHOO.augment`. See the history of augmentProto for background and recommendations for syntax.

In this example, a class has been created then augmented with `YAHOO.util.EventProvider` to provide custom event functionality.

Click the Send button to send the instance of the new class a request.

## YAHOO.lang is packaged with the YAHOO Global Object

`YAHOO.lang` comes bundled with the YAHOO Global Object. To add the YAHOO Global Object, include the following in your markup:

<script type="text/javascript" src="http://yui.yahooapis.com/2.8.1/build/yahoo/yahoo-min.js"></script>

If you are using any other YUI component on your page, you should already have `YAHOO.lang` available.

## The example: Any class can be an EventProvider

This example creates a custom class, then augments it with `YAHOO.util.EventProvider` (functionality included in the YUI Event Utility). Using the packaged functionality of `EventProvider`, the code for `Foo` is able to focus on the functionality unique to its purpose.

YAHOO.namespace('example'); // Create a class Foo for some greater purpose YAHOO.example.Foo = function () { /* code specific to Foo */ // Add a custom event for the instance this.createEvent('interestingMoment'); } YAHOO.example.Foo.prototype.doSomething = function() { /* ..do something interesting... */ // Fire off the custom event this.fireEvent('interestingMoment'); } // Make the custom event stuff possible via augmentProto YAHOO.lang.augmentProto(YAHOO.example.Foo, YAHOO.util.EventProvider); var f = new YAHOO.example.Foo(); // Add some event listeners f.subscribe('interestingMoment', function () { var p = YAHOO.util.Dom.get('demo_p1'); p.innerHTML = 'I was notified of an interesting moment'; }); f.subscribe('interestingMoment', function () { var p = YAHOO.util.Dom.get('demo_p2'); p.innerHTML = 'I was also notified of an interesting moment'; }); // Add a listener to the button to call the instance's doSomething method YAHOO.util.Event.on('demo','click', function () { f.doSomething() });

## Composition, not inheritance

If `Foo` were a part of a class hierarchy, it would be improper to include `EventProvider` in the inheritance chain, since the purpose of the two are fundamentally different.

Unlike `extend`ed classes, the relationship between a class and the classes augmenting it is not an indication of type hierarchy. The intent of `augmentProto` is to aid in extracting nonessential behaviors or behaviors shared by many classes, allowing for a composition-style class architecture.

" alt="Diagram showing class hierarchy, highlighting has-a relationship"/>

This may appear similar to multiple inheritance, but it's not. `augmentProto` simply adds the public methods and members from one class prototype to another class prototype. Instances of the augmented class will not pass `instanceof` tests for the class(es) which augmented it.

function Foo() {} Foo.prototype.doSomething = function () { /* something */ }; function Bar() {} YAHOO.lang.augmentProto(Bar, Foo); var b = new Bar(); if (b instanceof Bar) {} // true if (b instanceof Foo) {} // FALSE

## A short history of augmentProto

`augmentProto` started its life as `YAHOO.augment` way back in version 0.12 of YUI.

In version 2.2.0, it was moved into the `YAHOO.lang` module. The alias `YAHOO.augment` was added for backward compatibility.

In version 2.3.0, `augment` became `augmentProto`. Another alias, `YAHOO.lang.augment`, was created for backward compatibility.

While these aliases should be around for a while, it's advisable to use `YAHOO.lang.augmentProto` going forward to ensure that your code is as future-compatible as possible.

## Configuration for This Example

You can load the necessary JavaScript and CSS for this example from Yahoo's servers. Click here to load the YUI Dependency Configurator with all of this example's dependencies preconfigured.

### YAHOO Global Object Examples:

- Type-Checking Your Data
- User Agent Detection
- Creating Class Hierarchies with `YAHOO.lang.extend`
- Creating a Composition-Based Class Structure Using `YAHOO.lang.augmentProto`
- Add Behavior to Objects or Static Classes with `YAHOO.lang.augmentObject`
- Combining Simple Data Sets with `YAHOO.lang.merge`

### More YAHOO Global Object Resources:

- User's Guide (external)
- API Documentation

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